Space hardware, power plant equipment particularly for the nuclear industry, and a number of other devices which must be designed to meet high standards of resistance to shock and vibration are typically analyzed on testing apparatus capable of independently producing controlled motion simultaneously along up to three perpendicular axes. The majority of known devices of this type generally consist of multiple hydraulic actuators attached at one end to a fixed surface or ground and at the other end to various surfaces of a table upon which the device to be tested is mounted. Operation of the hydraulic actuators creates movements of the table along up to three axes for simulating seismic loads or similar disturbances. Strain gauges, accelerometers, or like measuring elements are attached to the device placed on the table and monitor the response of the device to such loads using known techniques.
One of the major problems associated with known seismic testing apparatus is the presence of undesired, rotational, tilting, or torsional movement of the table about the axes of excitation, which introduces inaccuracies in excitation of the device placed on the table for testing. This problem is occasioned by the fact that when a device is placed on top of the table, the combined center of gravity of the table and device is typically above or offset from the lines of action or excitation of the linear hydraulic actuators mounted to the table. Thus, as the table is linearly oscillated along its three orthogonal excitation axes, the combined center of gravity of the table and device, which is offset relative to the excitation axes, creates moments about these axes, tending to rotate the table.
The solutions suggested in the prior art to avoid this inherent tendency of the vibrating table to tilt or rotate generally rely on electronic closed loop position control systems which operate on the basic premise of correcting or compensating for the rotational movement once it has occurred rather than preventing such movement initially. While specific features of known control schemes vary from system to system, such systems typically rely on the development of a positional error signal in response to unwanted rotational or torsional movement which is sensed by appropriate position sensor means. The positional error signal is fed to the actuators as an input for continuous correction of the non-linear, or rotational, motion about one or more axes. Illustrative of such prior art schemes are U.S. Pat. Nos. 3,800,588; 3,911,732; 3,913,389 to Larson; and 3,918,298 to Petersen, et al.
A problem with system of the foregoing type, in addition to the need for compensating electronic controls, is that they inherently require larger and/or more pumps, valves, cylinders, etc., which generally increases the expense and difficulty of accomplishing the end result. This problem is due to the fact that the actuators must perform the dual function of imparting the desired linear oscillatory movement to the vibrating table as well as correct undesired rotational or torsional movement.